Change /loki/api/v1/push JSON object to be consistent with other v1 paths

[rfratto]

In the API docs for the push path, the POST body still accepts a single string to refer to all of the labels (i.e., "{foo=\"bar\"}").

This way of referring to the label set has deprecated in all other v1 endpoints, replaced with a labels object: {"foo": "bar"}. Before 0.4.0 is released with the new endpoints, it would be good to change the push endpoint to make it consistent with the other ones.